In hearing aid systems of this type using at least two spaced apart microphones, it is known that, in the technique for controlling the directionality and beam forming, using multiple microphones, usually two microphones, the realization depends on the microphones being matched as closely as possible with respect to their time an phase relationship as well as their sensitivity, because beam forming techniques make use of the time/phase difference between spaced apart microphones with respect to the direction of the sound received from a sound source.
The difference in the arrival time of signals at the microphone determines at which angles, the zeros in the directional characteristic will be generated.
Any disturbance in this difference in arrival time will disturb the position of the zeros in space, and the directional behaviour will, in this case, never become optimal.
Differences in the sensitivity between hearing aid microphones of the same type could be as large as 6 dB, which would result in a directional behaviour, that--for practical use--is not even there.
The difference in phase could be as large as 10.degree. at low frequencies, which is due to production tolerances in connection with the lower cut-off frequency in the microphones.
In hearing aids with preferably two microphones they will be normally placed apart by a distance of 1 cm. This corresponds to an acoustical delay between the microphones of about 30 .mu.s. Disturbances in the arrival times could of course be very severe, because they could in fact be larger than the actual acoustical delay between the two microphones.
A way to overcome this problem has until now been to use microphones, which were matched in their sensitivity and phase by the supplier.
However, there are some drawbacks in this method:
1. Microphones can not be matched better in their sensitivity by the supplier than to about 0.5 dB. However, o,5 dB is enough to degrade the directional behaviour heavily at 200-300 Hz. PA1 2. Microphones can not be matched better in phase than about 2.degree., because of the needed precision in the equipment used to measure the microphones. 2.degree. corresponds at 200 Hz to about 28 .mu.s, which in many cases is enough to move the directional characteristic, so that directions which were actually intended to be damped, remain almost undamped, and therefore will be transmitted with the same strength as the signal coming from the desired direction. PA1 3. The two electrical inputs in the hearing aid need to be matched as well, for the beam forming to work well. This implies a special selection of the components to be used, because the tolerances of e.g. capacitors are not sufficiently narrow. PA1 4. In case of one microphone or other components becoming defective, it will be necessary to exchange all microphones (or other components) as matched sets which will make the necessary service operation much more expensive.